In a PET method, a labeled compound labeled with a short-lived radionuclide that emits positron (hereinafter, referred to as a “tracer”) is administered into a living body, γ rays generated by this tracer are measured by using a PET camera (a detector including a gamma-ray scintillator and a photomultiplier), and the distribution thereof in the body is imaged by using a computer. This PET method is used in identification of a site of a tumor such as a cancer cell as an examination of nuclear medicine; a diagnosis of Alzheimer disease, brain infarction, and the like; a diagnosis of a mental disorder such as depression; an evaluation of treatment; and an evaluation of pharmacokinetics and drug efficacy.
As a short-lived radionuclide used in a PET method, 11C and 18F are well used. Since a 11C-labeled tracer uses a carbon atom existing in all organic compounds, it is applicable in an extremely wide range and becomes an ideal radionuclide. Furthermore, a method of preparing a compound such as 11CH3I, 11CO or 11CO2 serving as a precursor for synthesizing the 11C-labeled tracer is well established. Therefore, purified precursors can be stably obtained. Therefore, it can be said that the 11C-labeled tracer is an excellent tracer used in the PET method.
However, since the half-life of 11C is only about 20 minutes, it is said that labeling reaction with 11C including synthesis, purification and administration to a living body must be carried out within 40 minutes (within twice of the half time). Therefore, various methods for synthesizing PET tracers by using 11C for a short time have been considered.
In order to synthesize a PET tracer, hitherto, a methyl group labeled with 11C has been tried to be introduced into a heteroatom such as O, N, S or the like. In this method, however, since 11C is bonded to heteroatoms such as O, N, S and the like that are unstable to metabolism, a 11C methyl group may be released by metabolism before it reaches to a target organ in a living body. Therefore, it may not be possible to carry out a diagnosis and to evaluate treatment accurately.
In contrast, when a 11C-labeled methyl group is directly bonded to a carbon atom of an organic compound, the following advantages can be obtained.
Firstly, since a methyl group is a three-dimensionally smallest and non-polar functional group, the effect of the methyl group on the bioactivity of the parent compound can be minimized after introduction.
Secondly, since a 11C-labeled tracer has a short half-life of about 20 minutes, it is possible to carry out a large number of trials and clinical tests in a day. Furthermore, it is not necessary to pay a particular attention to processing of radio-labeled byproducts generated after a synthesis reaction, and the like.
Thirdly, a C-methylated product shows higher stability to metabolism as compared with an O-methylated product or an N-methylated product, and in the C-methylated product, 11C methyl can be introduced into parent nuclei of carbon of an important compound such as a metabolism-related compound or antimetabolite. As mentioned above, when a metabolism-related compound, antimetabolite or the like in which a parent nuclei structure of carbon is labeled with 11C methyl in a stable form is used, it is possible to more accurately evaluate diagnosis and treatment of various diseases.
A method of synthesizing a 11C-labeled compound is described in the following documents.
That is to say, patent document 1 discloses a method of producing 11C-labeled methyl iodide. The method includes the step of mixing carbon dioxide and hydrogen under pressure so as to form a first mixture, a step of passing the first mixture through catalyst so as to generate methanol, and a step of passing the methanol through an iodination reagent so as to be formed into methyl iodide.
Furthermore, patent document 2 discloses a method of synthesizing 11C-L-methionine by converting 11C methane thiol into 11C-L-methionine via an enzyme reaction of γ-cyano-α-aminobutyric acid synthase.
Furthermore, patent document 3 discloses a method of synthesizing 11C-labeled phosgene from 11C-labeled carbon tetrachloride by using iron oxide.
Furthermore, patent document 4 discloses a method of synthesizing a 11C-labeled halogenated methyl in which LiAlH4 dissolved in an organic solvent is introduced into a thin tube; an inert gas is allowed to pass through the solution so as to evaporate the solvent and a thin film of the remaining LiAlH4 is formed on a internal surface of the thin tube; then 11CO2 gas is introduced so as to be reacted with the remaining LiAlH4; and further a halogenated hydroacid or halogenated hydrogen gas is introduced so as to produce 11CH3X (wherein X represents a halogen atom) in the thin tube.
Furthermore, patent document 5 discloses a method of obtaining fluorine-labeled DOPA by a solid phase method in order to rapidly synthesize a PET tracer monitoring dopamine metabolism of the cerebrum by using 18F having a half life of 110 minutes. That is to say, in this method, in order to shorten the purification process of the synthesized labeled compound, 18F-labeled 6-L-fluoro DOPA is obtained by adding fluorine to DOPA protected by dimethyl tin and linked to a solid support via a linker.
The methods of preparing a PET tracer using 11C described in the above-mentioned patent documents have still required a long time for synthesis for preparing the PET tracer and have not provide satisfactory yield and purity. Therefore, these methods are not sufficient for carrying out diagnosis or study of pharmacokinetics reliably. Therefore, a method of introducing 11C radio nucleus has been demanded which is a higher-speed reaction and has a higher yield.
Under such circumstances, the present inventors have developed a high-speed methylation method of subjecting methyl iodide and an organic tin compound to a Stille-coupling reaction and received much attention (non-patent document 1). This method has enabled a cross-coupling between carbon of an aromatic ring and SP3 carbon, which has been conventionally thought to be difficult. For example, when methyl iodide, an excessive amount of tributylphenylstannane, tri-o-tolylphosphine and unsaturated palladium are reacted with each other in the presence of a copper salt and potassium carbonate in a DMF solvent at 60° C. for 5 minutes, methylation proceeds in a yield of 90% or more. Many drugs and drug candidate compounds have an aromatic ring or an alkenyl group in its basic skeleton. Since this method enabling a cross-coupling between carbon of the aromatic ring and SP3 carbon permits introduction of a 11C-labeled methyl group into the aromatic ring, the kinetics of these drugs and drug candidate compounds in a living body can be clarified thereby. The present inventors have actually applied this method to a prostaglandin derivative tracer and succeeded in imaging a prostaglandin receptor in the human brain. Thus, the usefulness thereof has already been verified.
Furthermore, non-patent document 2 reports the synthesis of a 11C-labeled PET tracer using an organic boron compound. This method is classified in Suzuki-Miyaura reaction in which a compound wherein a benzene ring is bonded to a boron atom and 11C-labeled methyl iodide are subjected to cross-coupling while heating in a DMF solvent in the presence of Pd (dppf) Cl2 and potassium phosphate in a microwave so as to obtain a 11C-labeled toluene derivative.
On the other hand, 18F has a half life (110 minutes) that is five times or longer than that of 11C (about 20 minutes) and has an advantageous that it is possible to extend the time for preparing a PET tracer and a time for carrying out PET diagnosis. Therefore, a technology for cross-coupling between an aromatic compound or an alkenyl compound and an 18F-labeled fluoromethyl group has been demanded. However, a method of modifying an aromatic compound or an alkenyl compound with an 18F-labeled fluoromethyl group rapidly and in a high yield has not been known.    [Patent document 1] JP-H07-165630 A    [Patent document 2] JP-H11-332589 A    [Patent document 3] JP-2002-308615 A    [Patent document 4] JP-2005-53803 A    [Patent document 5] JP-2005-502617 A    [Non-patent document 1] Chem. Eur. J. 1997, 3 (12), 2039-2042    [Non-patent document 2] Journal of Labelled Compounds and Radiopharmaceuticals 48, 629-634 (2005)